My understanding is that if NW can access your e-ticket or get possession of your paper ticket it has VALUE, and there is some sort of airline clearinghouse process (similar to the clearinghouse process for banks) whereby NW will get $ (revenue) from your ticket/flight coupon if they fly you. Every ticket, electronic or paper, goes through this clearinghouse process.

It's probably more complex than what I have written above, but that's my redneck Cliffs Notes version.

There is a ton of information over in the NW Forum on this, and I believe there is even an extensive thread here in the AA Forum addressing some aspects of the program as well. Try doing a search for "BUMP Program" in either forum and you'll be captivated for hours by all of the minute details of the program (courtesy of your fellow knowledgable FTers) that are available here on FT!